1. Field of the Invention
The present invention relates generally to the field of dentistry and more particularly to rotating or oscillating cutting instruments and tools useful for the removal of tooth structure, including decayed or damaged nerve tissues and dentine material on the interior walls of the root canal and dentine and enamel from the external tooth wall.
2. Description of the Related Art
In the field of endodontics, one of the most important and delicate procedures is that of cleaning or extirpating a root canal to provide a properly dimensioned cavity while essentially maintaining the central axis of the canal. This step is important in order to enable complete filling of the canal without any voids and in a manner which prevents the entrapment of noxious tissue in the canal as the canal is being filled.
In a root canal procedure, the dentist removes injured tissue and debris from the canal prior to filling the canal with an inert filling material. In performing this procedure the dentist must gain access to the entire canal, shaping it as necessary. But root canals normally are very small in diameter, and they are usually quite curved. It is therefore very difficult to gain access to the full length of a root canal.
Many tools have been designed to perform the difficult task of cleaning and shaping root canals. Historically, dentists have used a wide multitude of tools to remove the soft and hard tissues of the root canal. These tools, usually called endodontic files, have been made by three basic processes. In one process, a file is created by twisting a prismatic rod of either square or triangular cross section in order to create a file with helical cutting/abrading edges (“K-file”). The second process involves grinding helical flutes into a circular or tapered rod to create a file with one or more helical cutting edges (“Hedstrom file”). The third method involves “hacking” or rapidly striking a circular or tapered rod with a blade at a given angle along the length of the rod, thus creating an endodontic file characterized by a plurality of burr-like barbs or cutting edge projections (“barbed file” or “broach”). Each of these methods produces an instrument having unique attributes, advantages, and disadvantages.
Endodontic files have historically been made from medical-grade stainless steels. But steel is inherently stiff and brittle and, thus, these tools can sometimes pose a significant danger of breakage in the curved root canal, particularly if over-torqued or fatigued. Modern endodontic instruments are often formed from exotic alloys such as nickel-titanium commonly known as “Nitinol™” or “NiTi.” A series of comparative tests of instruments made of nickel-titanium alloy and stainless steel were conducted and published in an article entitled “An Initial Investigation of the Bending and the Torsional Properties of Nitinol Root Canal Files,” Journal of Endodontics, Volume 14, No. 7, July 1988, pages 346-351. The reported tests demonstrated that the NiTi instruments exhibited superior flexibility and torsional properties as compared to stainless steel instruments.
Based on the initial success of these and similar studies, NiTi endodontic instruments have been commercially introduced and have become widely accepted in the industry. As the use of such NiTi instruments has proliferated, however, certain drawbacks have become apparent. One particularly well-documented drawback is the expense and difficulty of machining endodontic files from NiTi alloy. Slow grinding with fine-grit grinding wheels is the presently accepted method for machining NiTi alloys. But, even then, special procedures and parameters must typically be observed to reliably obtain clinically acceptable instruments. See, for example, U.S. Pat. No. 5,464,362 to Heath et. al., which describes a method of slow grinding a rod of a nickel-titanium alloy to create a fluted endodontic file. The cost of purchasing and operating the required specialized 6-axis grinding machines and other grinding/machining equipment and the time consumption of the grinding process itself make endodontic files produced by this method inordinately expensive when compared to their stainless steel counterparts.
Another significant drawback is the extreme tendency of the NiTi material to form latent burrs, rolled metal deposits and/or other imperfections along the desired cutting edges during the machining process. If these imperfections are not carefully monitored and controlled, they can have deleterious effects on file performance. Another significant drawback is that the cutting edges of presently available NiTi instruments are not as sharp as their stainless steel counterparts and tend to lose their sharpness more rapidly with use. Another significant drawback is reduced manipulation control due to reduced stiffness (excessive “rubberiness”) and extreme torsional flexibility of presently available NiTi endodontic files as compared with stainless steel files. Another drawback is increased heat generation created by bare or oxidized NiTi surfaces rubbing against root canal walls.
These and other drawbacks have limited the growth of NiTi instruments and have created demand for improved manufacturing methods, alloys and instruments that overcome the aforenoted drawbacks.